Disco colors showing when either VS or language server refer to out-of-date `project.razor.vs.bin`

[maryamariyan]

Steps to repro

• change TFM in the project file, and save
• open App.razor and observe disco colors

Summary

This summary explains in a bit more detail a cause for why/when we end up seeing disco color in @ code blocks in razor files.

Latest observations suggest that:

• VS and the language server both rely on project.razor.vs.bin file which gets created in the obj folder.
  • disco color bugs happen when VS and language server end up looking at project.razor.vs.bin file in two separate obj folders
  • (and when one of the two ends up looking at an out of date / incorrect file)
• With the bug explained in
  • Razor Language Server finds old obj folders to watch, causing issues · Issue #9600 · dotnet/razor (github.com)
  • which is fixed in PR: https://github.com/dotnet/razor/pull/9601
  • with the fix, the language server no longer reads the out-of-date file
    • it assumes that VS is updating the new file

But for the current issue,

• knowing that opening a razor file or creating a new razor component triggers creation or update of project.razor.vs.bin file (when not created yet) we are noticing that
  • once we change target framework in csproj (e.g. from net6 to net8)
  • assuming the project.razor.vs.bin file does not exist yet in either net6 or net8 obj Debug folders, then:
    • if we open an existing razor file in VS, that causes the file creation in the old obj path
      • causing disco colors
    • but if we create a new razor file in VS, that causes the file creation in the new obj path
      • causing no disco colors